Field of the Invention
The present invention relates to a process for producing a polyolefin resin-coated paper which is used as a photographic support and more particularly to prevention of deterioration in surface quality due to contamination of a die lip which is caused upon preparation of a photographic support by melt-extruding a polyolefin resin composition containing titanium dioxide into a film state and coating such onto at least one surface of a paper sheet or a synthetic paper base.
A polyolefin resin-coated paper for photographic use is already known and a photographic polyolefin resin-coated paper as disclosed in, e.g., U.S. Pat. No. 3,501,298 comprises coating both surfaces of a paper base with polyethylene resin which contains titanium dioxide, blue pigments, fluorescent whitening agents, etc. in a polyethylene layer on the emulsion-coated side.
However, there is a tendency that needle-like or icicle-like attaches or stains (hereafter simply referred to as "die lip stains") generate at the top of a die lip by extrusion in a short period of time upon melt-extrusion of a polyolefin resin composition containing titanium dioxide, particularly a polyethylene resin composition containing titanium dioxide from a slit die in a film shape and, to be difficult to deal with, there is a tendency that these dye lip stains grow more and more with the passage of time for melt extrusion.
If the dye lip stains generate upon coating by melt-extrusion, streaks are formed on the surface of a photographic support in the longitudinal direction thereof, when the support is prepared in that state, or streakened uneveness in coating is formed due to an uneven coated amount, or stains are sometimes attached and coated onto a film to thereby produce foreign matters. For these reasons, surface quality of a polyolefin resin-coated paper thus prepared is seriously damaged and quite inadequate and of no commercial value for use of a photographic support which requires excellent surface quality.
Further, for completely removing die lip stains once formed, there is nothing but discontinuing the production line and cleaning a die lip; hard labors and time are required for the cleaning and such results in serious reduction in producibility. Thus immediate solution of such a problem has been desired.
An object of the present invention is to provide a process for producing a polyolefin resin-coated paper for photographic use capable of preventing deterioration in surface quality due to die lip stains which generate upon preparing a polyolefin resin-coated paper for photographic use.
As a result of extensive investigations on causes for generating the die lip stains, the present inventors have found that the die lip stains are seriously affected by titanium dioxide contained in the polyolefin resin composition. That is, noting the content of TiO.sub.2, if the content of TiO.sub.2 in the polyolefin resin composition is decreased, die lip stains generated are reduced but a shielding power becomes poor so that the product cannot be used as a photographic support. On the other hand, if the content of TiO.sub.2 is increased, a shielding power is sufficient but it has been found that die lip stains seriously occur.
It has further been found that die lip stains are affected not only by the addition amount of titanium dioxide but also by titanium dioxide per se employed.
In the case where no titanium dioxide is incorporated into a resin layer of a photographic support, of course, sharpness is seriously deteriorated and the support is useless as a photographic support.
Titanium dioxide conventionally used in the photographic art is titanium dioxide which is modified by depositing hydrous aluminum oxide or hydrous aluminum oxide and hydrous silicon dioxide onto the particle surface of titanium dioxide as disclosed, for example, in Japanese Patent Application KOKAI No. 35625/77 (The term "KOKAI" refers to an unexamined application open to public inspection.), etc.
Reasons way titanium dioxide to which surface treatment has been made to such a high extent is employed in the photographic art are because, as are also described in Japanese Patent Application KOKAI No. 35625/77, in the case of using titanium dioxide which has undergone no surface treatment or surface treatment to a very mild extent (an amount of a surface treating agent is less than 0.2 wt % based on titanium dioxide):
(1) This titanium dioxide adversely affects photographic emulsion. PA1 (2) Dispersibility of titanium dioxide becomes poor. PA1 (3) Adhesion between a resin layer and an emulsion layer becomes poor. PA1 (4) White background of a photograph is liable to undergo change with the passage of time. PA1 (5) Whitening effect with a fluorescent whitening agent is poor.
Hardly is thus such titanium dioxide used in a resin layer of a photographic support requiring particularly severe quality that has undergone no surface treatment or surface treatment to a very mild extent. However, as titanium dioxide is subjected to surface treatment to such a high extent for satisfying the requirement in the art, occurrence of die lip stains becomes serious.
As a result of extensive investigations on simultaneous solution of these contradictory problems, i.e., problem in quality such as liability of undergoing color change at the white background with the passage of time (hereafter referred to as "photodeterioration") and problem on operation due to serious occurrence of die lip stains (which leads to poor quality as a result), the present inventors have reached the present invention.